1. Field of the Invention
This invention relates generally to liquid level sensors and alarms for containers of liquids, and more particularly to a sensor and alarm useful in the holds of barges and the like.
2. Description of the Prior Art
It is important that large containers of liquid not be overfilled, for several reasons. In the past, it was acceptable practice to leave open the top access hatches in large containers such as the holds of tankers and barges, particularly during filling operations, to avoid generation of pressures in the holds during the filling operations as the volume of liquid content increases and the remaining tank volume for vapor decreases. The vapor was simply permitted to escape to atmosphere. Because the vapors of many types of materials are considered environmentally objectionable, even if not particularly dangerous in the sense of flammability or corrosive nature, governmental authorities have imposed restrictions such that the holds of vessels must be sealed closed during filling operations. Accordingly, if provisions made for removing vapor during filling operations are inadequate, the pressure in the hold will rise. Many, if not most, large volume liquid cargo tanks cannot withstand very significant internal pressures. For some of them, three pounds per square inch is the maximum tolerable internal pressure.
Although spill valves have been employed to enable contents to spill out from openings in the top of such tanks if they are filled excessively, or if the vapor pressure rises too much, it is preferable to employ some means to determine the amount of fill in such a tank. One method for doing so has been to employ a cylindrical float in the tank and which rises and falls with the level of the liquid therein. As the float rises and falls, it is centered and guided on an upstanding cylindrical pipe fixed in the tank. A magnetic reed switch is mounted inside the pipe Magnet means of one or more varieties are mounted in the float so that, if the float rises to a certain level it will close the reed switch and turn on some kind of signaling device. There are several disadvantages with this type of system. The float performance is influenced by the viscosity of the material in the tank and/or any accumulation of dirt, foreign matter, degraded cargo content or other impediment which may have accumulated either on the guide hole in the float or on the guide pipe itself. Due to the gap necessary (although small) between the pipe and the float to permit the float to reasonably freely move up and down on the pipe, and since the reed switch is inside the pipe and the magnet is inside the float, and due to the wall thicknesses of the pipe and float, the distance between the magnet and the reed switch is fairly significant. Consequently, a comparatively large magnet must be used to properly operate the switch. But it is possible that prolonged influence of the comparatively large magnet on the switch will magnetize the switch so that it will remain actuated even when the float has moved away from the switch.
With the above-mentioned conventional system, testing in the field is usually done by means of a test rod which engages the float and lifts the float to see if the switch is operated at some point as the float is lifted. But it is difficult or impossible to test the assembly for performance of the float itself. It may appear to be functioning properly when using the test rod; however in actual conditions the device may be inoperative for many reasons. The float may have enough gummy material between it and the shaft that it cannot rise due to the bouyancy alone, although the test rod will move it. The float may have a hole in it so that it is actually sunk and has no bouyancy. The float may be partially filled with fluid and will rise too late. The present invention is directed to overcoming one or more of the above-mentioned disadvantages of the prior art.